Mitochondrial disease can present with a wide range of clinical phenotypes, and knowledge of the clinical spectrum of mitochondrial DNA mutation is constantly expanding. Leigh syndrome (LS) has been reported to be caused by the m.13513G>A mutation in the ND5 subunit of complex I (MT-ND5 m.13513G>A). We present a case of a 12-month-old infant initially diagnosed with tachyarrhythmia requiring defibrillation, subsequent presentation with hypertension and hyponatraemia secondary to renal salt loss and presumed inappropriate ADH secretion. Complex I activity in the muscle tissue was 54%, and mutation load in the muscle and lymphocytes was 50%. This case of Leigh syndrome caused by the m.13513G>A mutation in the ND5 gene illustrates that hyponatraemia due to renal sodium loss and inappropriate ADH secretion and hypertension can be features of this entity in addition to the previously reported cardiomyopathy and WPW-like conduction pattern and that they present additional challenges in diagnosis and management.